1. Field of Invention
The present invention relates to an improved probe. The probe has been designed in particular but not exclusively for measuring the concentration of live biomass.
2. Background and Related Art
Capacitance measurement techniques are known for measuring the capacitance (or specific capacitance or dielectric constant) of liquids and suspensions, such as biological cells in ionic aqueous solutions.
Monitoring systems incorporating such measurement capability are beneficial for measuring concentration of live cells. In particular in the brewing industry, the concentration of live yeast can be measured with an on-line capacitance probe. A radio frequency applied from the electrodes of the probe causes ions in the suspending medium (for example wort or green beer) and the cytoplasm of the yeast to move towards the two respective oppositely charged electrodes. As the plasma membrane is non-conducting a buildup of charge results in the cells and are said to be polarized with the yeast cells acting as tiny capacitors within the medium. Non-viable cells or cells with a damaged membrane do not interfere with the signal. Thus, a buildup of charge cannot occur as the ions can freely move across the membrane and so the cells do not become polarized. The measured capacitance is directly proportional to the amount of viable yeast within a sample over a wide concentration range. Such technology can also be utilized for measuring biomass in the field of biotechnology, for example, in controlling cell culture processes.
A variety of different probe arrangements have been utilized. One probe utilizes four electrodes projecting in the longitudinal length of the probe. These four electrodes project into the medium to be measured. The reason that four electrodes are utilized is that the current can be measured between a first pair of electrodes (usually the outermost electrodes) connected to a power input and the other two electrodes (second pair) can be utilized to measure the voltage across them via a high impedance volt meter such that there is virtually no current across this second electrode pair. The voltage typically is not measured across the electrodes through which the current passes due to the significant effect of polarization impedance meaning that the voltage could not be measured accurately. Polarization impedance is the impedance between the electrode and the medium to be measured, which is effectively in series with the impedance of the medium to be measured, thus leading to a distorted and in accurate result for measurement of the medium impedance.
An alternative probe is one where again four electrodes are provided but in this embodiment they are provided such that they do not project into the medium to be measured but instead sit flush against a probe surface and generally extend perpendicular the longitudinal length of the probe. Such a probe is shown in FIG. 1. Referring to FIG. 1, the flange portion 2 seats adjacent an opening in a measurement container such that the electrodes 4 are positioned in the measurement container.
Referring to FIG. 2 an alternative probe arrangement is shown comprising annular electrodes 4 provided within a probe body 6. In this embodiment the probe extends into the measurement chamber. There are advantages associated with the arrangement of FIG. 2 in that the effect of bubbles is reduced. There is still, however, as with the other prior art embodiments described, a problem with respect to polarization between the electrode and the liquid to be measured, which will be measured in series with the measure impedance of the liquid. This is important as the impedance required is the impedance of the liquid without the unwanted influence of the impedance between the electrode and the liquid. In order to reduce this effect the spacing between the electrodes in FIG. 2 is increased which makes the impedance between the electrode and the liquid less significant compared to the impedance of the liquid. Accordingly, the significance of electrode/liquid impedance is reduced, however, there still remains a problem in that the separation of electrodes is determined by the probe size whereby it is desirable to reduce the size of the probe to reduce manufacturing costs and also to reduce the size of the necessary monitoring equipment, thereby enabling a reduced sample size of the testing medium.
A further way of reducing the polarization impedance effect is to use electrode materials such as gold or platinum, however, although this is beneficial the effect of polarization impedance remains.
The present invention provides an improved probe arrangement that is less susceptible to the effect of polarization impedance between the electrodes and the liquid and also enables a significant reduction in probe size.